Energy Dissipation and Recovery in the Context of Silicon Production: Exergy Analysis and Thermoelectricity

Abstract

The metallurgical industry is power intensive and produces not only metals but also large amounts of excess heat. To increase the energy efficiency in this industry is thus of major interest, from both an environmental and an economical point of view. Exergy analyses were carried out for two industrial silicon furnaces (A and B), in operation at two different plants, and for a theoretical description of the silicon furnace. Furnace A was evaluated for two choices of carbon raw material mixtures and furnace B was assessed in the absence and presence of power production from waste heat in the off-gas system. The theoretical process was used to estimate what high exergy efficiency means in numbers and to explore the characteristics of the exergy flow in such an ideal process. The overall exergy efficiencies were 0.30-0.33 for the industrial silicon furnaces with no power recovery. For furnace B, the overall exergy efficiency was estimated to increase from 0.33 to 0.41 with power recovery from thermal energy in the offgas. The silicon yield exergy indicator is a measure for the performance of furnace operation that coincides with the specific power consumption. The theoretical process provides the upper limit of 0.51 for this indicator while for the industrial furnaces (no power recovery) values were 0.38-0.41. Additional exergy introduced as volatiles and through the consumption of electrodes accounted for 8-11 % of the total exergy destruction in the industrial silicon furnaces. Exergy destruction due to combustion of by-product gases, exergy lost with the furnace off-gas and ohmic losses in the power supply system were other major contributors to the thermodynamic inefficiency of silicon furnaces. Thermoelectric generators are scalable, simple and adaptable systems for power generation from various heat sources. A 0.25 m2 thermoelectric generator (TEG), based on bismuth-tellurium technology, was constructed and implemented at a silicon plant to study power generation from heat available during casting of silicon. The on-site measurements and a mathematical model were combined to explore the potential further. The measured peak power was 160Wm−2 and the corresponding maximum temperature difference across the modules was 100 K. For a twofold increase of the heat transfer coefficient on the cold side, and by moving the generator closer to the heat source, the power output was predicted to reach 900 W m−2. By tailoring the design of the TEG to the conditions encountered in the industrial facility, it is possible to generate more power with less thermoelectric material. Guidelines were provided on how to design thermoelectric systems to maximize the power generation from waste heat released from silicon during casting. Electrochemical cells with molten carbonates and gas electrodes were studied theoretically and experimentally for recovery of heat in the temperature range 400- 800 ◦C. The theory of non-equilibrium thermodynamics was used to describe the energy conversion in the system. The measured Seebeck coefficients of these cells were in the range 0.88 to 1.25 mV K−1. The larger value applies to a nearly equimolar mixture of lithium and sodium carbonate in dispersion with magnesium oxide. These cells can also exploit off-gases with concentration of carbon dioxide different from air, by adding a concentration cell operating on off-gases and air. The series construction has the potential to offer a power density at matched load conditions in the order of 0.5 kW m−2. The exergy analyses mapped the current status of the major exergy flows, exergy destruction and exergy losses in the silicon furnaces. These analyses quantified the potential for (thermodynamic) improvements, contributed to the fundamental understanding of energy conversion in the silicon furnaces and gave estimates for upper limits to exergy efficiency in this process. Thermoelectric generators are a class of systems with potential for recovery of heat from metallurgical processes. The cheap materials and the large Seebeck coefficient, in combination with a good overall electric conductivity, make the molten carbonate cells suitable candidates for electrochemical energy conversion in the future.